1. Field of the Invention
The present invention relates to circuitry for decoding multi-channel TV sound signals according to BTSC system specification, and to a multi-channel TV sound decoder comprising such circuitry.
2. Description of the Related Art
In the United States, the multi-channel television sound (MTS) system that is used for television broadcasting conforms to the Broadcasting Television System Committee (BTSC) standard (see OST Bulletin No. 60 (Revision A): Multi-channel sound transmission and audio processing requirements for the BTSC system (February 1986)). The MTS system is similar to the FM stereo system used in radio broadcasting, and involves a multiplex signal including a sum channel, a pilot tone, and a difference channel that is AM-modulated at twice the pilot frequency, with an additional monophonic channel (second audio program, or SAP, channel) being frequency-modulated at a center frequency of five times the pilot tone frequency. This composite signal is frequency-modulated before transmission. At the receiver, the demodulation process produces a parabolic noise power density at the output of the FM-demodulator, resulting in worse difference channel and SAP signals compared to the sum channel signal.
Excessive degradation of the difference channel and the SAP channel in the BTSC system is avoided by using a level- and frequency-dependent compression of these two sensitive channels at the transmitter; the expander inside the receiver must restore the original signal. The companding process, in which the signal is compressed on input and expanded back to its original form on output, attenuates the noise in silent periods; in loud periods, the noise is masked by the signal.
In prior analog expander circuitry employed in receivers, stereo channel separation adjustment involved a critical and time-consuming alignment procedure to minimize crosstalk. In the current implementation of digital circuitry, such adjustment is superfluous, since all functions behind the main FM demodulator in the BTSC system have been processed digitally. Digital decoder systems of illustrative conventional type are described in U.S. Pat. No. 6,281,813, and U.S. Pat. No. 6,492,913.
The digital implementations of the BTSC system that have been developed to date are based on a combination of an analog-to-digital converter (A/D converter) and a digital signal processor (DSP) as shown in the BTSC decoder system schematically illustrated in FIG. 1 (PRIOR ART). In the FIG. 1 system, the output signal of an antenna passes to the tuner. The tuner responsively generates a Sound Inter-carrier Frequency (SIF) signal, which is a carrier signal at a frequency of 4.5 MHz that is frequency-modulated with a Multi-channel Television Sound (MTS) signal. The SIF signal, as illustrated, then enters the digital frequency modulation demodulator unit and passes to the A/D converter, in which the SIF signal is converted to a digital SIF signal. The resultant digital SIF signal then is post-processed in the DSP of the digital frequency modulation demodulator unit, to produce a digital MTS signal.
Since the digital SIF signal is a frequency-modulated signal, it can be demodulated in various ways in the digital frequency modulation demodulator unit shown in FIG. 1.
In one exemplary digital FM demodulation arrangement, schematically illustrated in FIG. 2 (PRIOR ART), the output of the A/D converter is processed in an in-phase/quadrature-phase demodulator, wherein the quadrature I and Q base-band samples are calculated by the DSP with the digital SIF signal.
In another exemplary digital FM demodulation scheme, the DSP carries out the calculation using an arc-cosine look-up table to convert the digital SIF signal into a digital base-band composite signal.
In both of the foregoing exemplary digital FM demodulation schemes, the data of the SIF signal sampled by the A/D converter are desirably normalized before calculation is carried out by the DSP.
The above-described digital decoder systems require that an amplitude value of the SIF signal should be appropriately adjusted within a full-scale range of the A/D converter. Such adjustment can be effected manually during manufacture of the digital decoder system, or it can be done automatically with additional circuitry components, e.g., an auto-gain-controlled amplifier.
Thus, the digital FM demodulator including an A/D converter and a DSP still requires adjustment. In order to avoid the need for manual adjustment, additional circuitry is required, which can significantly increase the cost and complexity of the receiver, and reduce its reliability.